Fabrication and processing of polymer solar cells: A review of printing and coating techniques

Following the development of the bulk heterojunction1 structure, recent years have seen a dramatic improvement in the efficiency of polymer solar cells. Maximizing the open-circuit voltage in a low-bandgap polymer is one of the critical factors towards enabling high-efficiency solar cells. Study of the relation between open-circuit voltage and the energy levels of the donor/acceptor2 in bulk heterojunction polymer solar cells has stimulated interest in modifying the open-circuit voltage by tuning the energy levels of polymers3. Here, we show that the open-circuit voltage of polymer solar cells constructed based on the structure of a low-bandgap polymer, PBDTTT4, can be tuned, step by step, using different functional groups, to achieve values as high as 0.76 V. This increased open-circuit voltage combined with a high short-circuit current density results in a polymer solar cell with a power conversion efficiency as high as 6.77%, as certified by the National Renewable Energy Laboratory. Adding electron-withdrawing groups to the backbone of the polymer PBDTTT is shown to increase the open-circuit voltage of photovoltaic cells, resulting in a polymer solar-cell that has a certified power-conversion efficiency of 6.77%.

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Polymer solar cells with enhanced open-circuit voltage and efficiency

https://orbit.dtu.dk/files/7590153/Roll_to_roll_fabrication.pdf

Roll-to-roll fabrication of polymer solar cells

Organic photovoltaics (OPVs) evolve in an exponential manner in the two key areas of efficiency and stability. The power conversion efficiency (PCE) has in the last decade been increased by almost a factor of ten approaching 10%. A main concern has been the stability that was previously measured in minutes, but can now, in favorable circumstances, exceed many thousands of hours. This astonishing achievement is the subject of this article, which reviews the developments in stability/degradation of OPVs in the last five years. This progress has been gained by several developments, such as inverted device structures of the bulk heterojunction geometry device, which allows for more stable metal electrodes, the choice of more photostable active materials, the introduction of interfacial layers, and roll-to-roll fabrication, which promises fast and cheap production methods while creating its own challenges in terms of stability.

Stability of polymer solar cells.

An inverted polymer solar cell geometry comprising a total of five layers was optimized using laboratory scale cells and the operational stability was studied under model atmospheres. The device geometry was substrate-ITO-ZnO-(active layer)-PEDOT:PSS-silver with P3HT-PCBM as the active layer. The inverted devices were compared to model devices with a normal geometry where the order of the layers was substrate-ITO-PEDOT:PSS-(active layer)-aluminium. In both cases illumination was through the substrate which requires that it is transparent. Both device types were optimized to a power conversion efficiency of 2.7% (1000 W m−2, AM1.5G, 72 ± 2 °C). The devices were operated under illumination while being subjected to different atmospheres to identify the dominant modes of degradation. Dry nitrogen (99.999%), dry oxygen (99.5%), humid nitrogen (90 ± 5% relative humidity) and ambient atmosphere (20% oxygen, 20 ± 5% relative humidity) were employed and both device types were found to be stable in a nitrogen atmosphere during the test period of 200 hours. The devices with a normal geometry where an aluminium electrode is employed gave stable operation in dry oxygen but did not give stable device operation in the presence of humidity. The inverted devices behaved oppositely where the less reactive silver electrode gave stable operation in the presence of humidity but poor stability in the presence of oxygen. The inverted model device was then used to develop a new process giving access to fully roll-to-roll (R2R) processed polymer solar cells entirely by solution processing starting from a polyethyleneterephthalate (PET) substrate with a layer of indium-tin-oxide (ITO). All processing was performed in air without vacuum coating steps and modules comprising eight serially connected cells gave power conversion efficiencies as high as 2.1% for the full module with 120 cm2 active area (AM1.5G, 393 W m−2) and up to 2.3% for modules with 4.8 cm2 active area (AM1.5G, 1000 W m−2).

Jette Kristensen

Papers

A complete process for production of flexible large area polymer solar cells entirely using screen printing—First public demonstration